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United States Patent |
5,612,634
|
MacKenna
|
March 18, 1997
|
Circuit for sensing whether or not an add-in board is inserted into a
bus connector of a mother board
Abstract
In an environment where a contact of a mother board is connected to a logic
HIGH level voltage through a strong pull-up resistor, and a corresponding
contact of an add-in board is connected to an open-drain driver, such that
a control line driven by the open-drain driver is provided through the two
contacts in normal mode operation when the two contacts make electrical
connection, a circuit to sense whether or not the contact of the add-in
board is making electrical connection with the contact of the mother board
includes a weak pull-down resistor connected at one end to a node
connecting the output of the open-drain driver to the contact of the
add-in board, and connected at the other end to a ground reference.
Accordingly, a voltage detected on the node is pulled up close to the
logic HIGH level voltage if the contact of the add-in board is making
electrical connection with the contact of the mother board and the
open-drain driver is turned off, and is pulled down close to ground if the
contact of the add-in board is not making electrical connection with the
contact of the mother board and the open-drain driver is turned off. The
voltage thus detected is provided to other circuitry on the add-in board,
and/or back to a host computer on the mother board.
Inventors:
|
MacKenna; Craig A. (Los Gatos, CA)
|
Assignee:
|
Zilog, Inc. (Campbell, CA)
|
Appl. No.:
|
311951 |
Filed:
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September 26, 1994 |
Current U.S. Class: |
326/62; 235/441; 326/82 |
Intern'l Class: |
H03K 019/017.5 |
Field of Search: |
326/21,62,82,86
235/441
439/59,62
|
References Cited
U.S. Patent Documents
4675769 | Jun., 1987 | Marshall et al.
| |
4885482 | Dec., 1989 | Sharp et al. | 326/47.
|
4948954 | Aug., 1990 | Dias | 235/441.
|
4990760 | Feb., 1991 | Tomari et al. | 235/441.
|
5162675 | Nov., 1992 | Olsen et al. | 326/101.
|
5266783 | Nov., 1993 | McAllister | 235/441.
|
Primary Examiner: Westin; Edward P.
Assistant Examiner: Santamauro; Jon
Attorney, Agent or Firm: Majestic, Parsons, Siebert & Hsue
Claims
What is claimed is:
1. A sensing circuit for sensing whether or not a first contact of a first
electronic module unit is electrically connected to a second contact of a
second electronic module unit, wherein said first electronic module unit
includes an open-drain driver connected to said first contact, and said
second electronic module unit includes a pull-up resistor connected
between said second contact and a logic level voltage, such that when said
first and second contacts are electrically connected, a voltage on the
connection of said first and second contacts is pulled up close to said
logic level voltage when said open-drain driver is turned off, and pulled
down to a ground reference voltage when said open-drain driver is turned
on, said sensing circuit comprising:
a pull-down resistor connected between said first contact and a ground
reference voltage, wherein said pull-down resistor has a resistance value
greater than a resistance value of said pull-up resistor of said second
electronic module, such that, when said first and second contacts are
electrically connected and said open-drain driver is turned off, a voltage
on a node defined at the connection between said pull-down resistor and
said first contact is substantially pulled up to said logic level voltage,
and when said first and second contacts are not electrically connected and
said open-drain driver is turned off, said voltage on said node is
substantially pulled down to said ground reference voltage; and
means for communicating a logic level indicative of said voltage on said
node defined at the connection between said pull-down resistor and said
first contact, said communicating means including an addressable tri-state
driver having an input connected to said node defined at the connection
between said pull-down resistor and said first contact, and an output
indicative of a voltage on said node defined at the connection between
said pull-up resistor and said first contact, wherein said first
electronic module unit has a third contact connected to said output of
said addressable tri-state driver, and said second electronic module unit
has a fourth contact electrically connected to said third contact of said
first electronic module unit, such that a host computer on said second
electronic module unit enables said addressable tri-state driver after
addressing said addressable tri-state driver.
2. A sensing circuit for sensing whether or not a first contact of a first
electronic module unit is electrically connected to a second contact of a
second electronic module unit, wherein said second electronic module unit
includes a pull-up resistor connected between said second contact and a
logic HIGH voltage, said sensing circuit comprising:
a pull-down resistor connected between said first contact and a ground
reference voltage, wherein said pull-down resistor has a resistance value
greater than a resistance value of said pull-up resistor of said second
electronic module unit, such that, when said first and second contacts are
electrically connected, a voltage on a node defined at the connection
between said pull-down resistor add said first contact is substantially
pulled up to said logic HIGH voltage, and when said first and second
contacts are not electrically connected, said voltage on said node is
substantially pulled down to said ground reference voltage; and
means for communicating to a circuit on said first electronic module unit,
a logic level indicative of said voltage on said node defined at the
connection between said pull-down resistor and said first contact, wherein
said communicating means includes a R-S latch having a high-active set
input connected to said voltage on said node defined at the connection
between said pull-down resistor and said first contact, a reset input
connected to a reset signal, and an output indicative of whether or not
said first contact of said first electronic module unit is electrically
connected to said second contact of said second electronic module unit.
Description
BACKGROUND OF THE INVENTION
This invention relates in general to computer systems having a mother board
with one or more bus connectors for receiving one or more add-in boards
and in particular, to a circuit for electronically sensing whether or not
an add-in board is inserted into a bus connector of the mother board.
FIG. 1 illustrates, as an example, a mother board 10 having a plurality of
integrated circuits, 14-1 to 14-3, for performing certain functionality of
the mother board 10, and two slots, 11 and 12, for optionally receiving
add-in boards which serve to expand the functionality of the mother board
10. So as to be compatible with the ISA or AT bus used in personal
computers, each slot may have either one or two connectors, wherein one
connector (e.g., 15 and 17) includes all of the signals needed for
transferring 8 bits of data at a time between a host computer on the
mother board 10 and an add-in board inserted into the connector, and the
second connector (e.g., 16), if provided, includes those additional
signals needed for transferring 16 bits of data at a time.
FIGS. 2 and 3 respectively illustrate, as examples, a first add-in board 20
having a plurality of integrated circuits, 26-1 to 26-3, for performing
certain expanded functionality, and a second add-in board 30 also having a
plurality of integrated circuits, 36-1 to 36-3, for performing other
expanded functionality. To make electrical contact with the mother board
10, the first add-in board 20 also includes two connectors, 22 and 24,
adapted to be inserted into either slot 11 or 12 of the mother board 10,
and the second add-in board 30 includes one connector 32 adapted to be
inserted into either slot 11 or 12 of the mother board 10. In particular,
connectors 22 and 24 of the first add-in board 20 are adapted to be
respectively inserted into connectors 15 and 16 of the mother board 10
when the first add-in board 20 is inserted into slot 11, and connectors 22
and 32, respectively of the first add-in board 20 and the second add-in
board 30, are adapted to be inserted into connector 17 of the mother board
10 when either the first add-in board 20 or the second add-in board 30 is
inserted into slot 12. The second add-in board 30 also includes an RS-232
connector 34 adapted to be connected to a serial port (not shown) on the
mother board 10.
FIG. 4 illustrates, as an example, a block diagram of a computer system
wherein the first add-in board 20 is inserted into slot 11 of the mother
board 10, and the second add-in board 30 is inserted into slot 12 of the
mother board 10. In particular, with the add-in boards, 20 and 30, so
inserted, a host computer 40 on the mother board 10 communicates with the
second add-in board 30 via an 8-bit bus 42, and communicates with the
first add-in board 20 via a 16-bit bus 44, wherein the 8-bit bus 42
includes signals transmitted through connector 17 and the 16-bit bus 44
includes signals transmitted through connectors 15 and 16 as previously
described, for example, with reference to the ISA or AT bus used in
personal computers.
FIG. 5 illustrates, as an example, a bus structure 400 typical of the 8-bit
bus 42 and 16-bit bus 44. In particular, the bus structure 400 includes a
plurality of data lines 52 (also referred to herein as a "data bus") and a
plurality of address lines 54 (also referred to herein as an "address
bus"), which may be separate or combined by conventional multiplexing
techniques. For an 8-bit bus, such as the 8-bit bus 42, the bus structure
400 would have 8 data lines, and for an 16-bit bus, such as the 16-bit bus
44, the bus structure 400 would have 16 data lines. Also included in the
bus structure 400 are a plurality of control lines 50, one or more ground
lines 46, and one or more power lines 48.
When the first add-in board 20 is inserted into slot 12 of the mother board
10, the second connector 24 of the add-in board 20 is left "hanging in
air" while the first connector 22 of the add-in board 20 is inserted into
the single connector 17 of slot 12. If the add-in board 20 is of a type
requiring connection to a 16-bit bus, then it should be pulled out of slot
12 and reinserted into a 16-bit slot, such as slot 11. 0n the other hand,
if the add-in board 20 is of a type which can operate in either 8-bit or
16-bit mode, then it may be desirable for the add-in board 20 to
automatically configure its operating mode to conform to the type of slot
it has been inserted into and ignore or block the unconnected (i.e.,
"hanging") signals so that they do not interfere with the operation of the
add-in board 20. In either case, it is useful for the add-in board 20 to
be able to automatically determine, as soon as possible after reset or
system start up, for examples, whether it has been inserted into an 8-bit
or a 16-bit slot.
It is also desirable at times for the second add-in board 30 to determine
whether or not it has been inserted into a slot of the mother board 10, as
opposed to being connected to a power supply and used in a stand-alone
manner. For example, the add-in board 30 may function as either an
internal or external modem depending upon whether or not it has been
inserted into a slot, such as slot 11 or 12, of the mother board 10. In
particular, when the add-in board 30 is inserted into slot 12, it
functions as an internal modem by communicating data through connector 17,
and when the add-in board 30 is not inserted into slot 12, it acts as an
external modem by communicating data through its RS-232 connector 34.
Accordingly, it may also be desirable for the add-in board 30 to
automatically determine, as soon as possible after reset or system start
up, for examples, whether it has been inserted into a slot of the mother
board 10.
FIG. 6 is useful for illustrating a prior art technique for determining
whether or not an add-in board is inserted into a bus connector of a
mother board. In the technique, one of the ground lines (e.g., 46-1 of the
ground lines 46) from the mother board is used for determining whether or
not the add-in board is inserted into the bus connector of the mother
board. A drawback of the technique is that the ground line used for such
determination is no longer available as a ground line to the add-in board
(although it continues to function as a ground line for other add-in
boards connected to it).
The ground line (e.g., 46-1) is connected from the mother board through a
pin or contact 27 of its bus connector (e.g., slot 11 or 12) to the add-in
board through an opposing pin or contact 57 of a mating connector of the
add-in board. On the add-in board, a pull-up resistor 56 is connected at
one end to the pin or contact 57 of the add-in board at node 58, and at
another end to a voltage, such as a logic level voltage of 5 volts. A
voltage detection circuit 60 detects the voltage V on the node 58, and
generates a control signal 61 indicative of the detected voltage. If the
add-in board is inserted into the bus connector of the mother board, then
opposing pins or contacts, 57 and 27, respectively of the add-in board and
the mother board, come together making electrical contact. As a result,
the voltage V at node 58 is pulled down close to ground "gnd", and the
detection circuit 60 generates a corresponding control signal 61, such as,
for example, a logic level LOW signal. On the other hand, if the add-in
board is not inserted into the bus connector of the mother board, then
opposing pins or contacts, 57 and 27, respectively of the add-in board and
the mother board, do not come together to make electrical contact. As a
result, the voltage V at node 58 is pulled up through resistor 56 close to
the logic level voltage of 5 volts, and the detection circuit 60 generates
a corresponding control signal 61, such as, for example, a logic level
HIGH signal.
As previously mentioned, one problem with such a prior art technique for
electronically detecting whether or not an add-in board is inserted into a
bus connector of a mother board, is that at least one of the ground lines
of the mother board must be dedicated with respect to the add-in board for
such detection and as a result, is not available for use as a ground line
by the add-in board. For high performance applications, this reduction in
the number of ground lines can be a significant disadvantage.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is a circuit for
electronically sensing or detecting whether or not two electronic module
units are connected together at their respective contacts, without
requiring the dedication of any of the two electronic module units'
contacts for such sensing or detecting.
This and additional objects are accomplished by the various aspects of the
present invention, wherein briefly stated, one aspect of the present
invention is a sensing circuit for electronically sensing or detecting
whether or not a first contact of a first electronic module unit is
connected to a second contact of a second electronic module unit without
requiring either the first or second contact to be dedicated for such
sensing or detecting. In particular, the first contact in this aspect of
the present invention, remains useful for transmitting signals from the
first electronic module unit to the second electronic module unit, and the
second contact remains useful for receiving the transmitted signals from
the first electronic module unit during normal operation of the first and
second electronic module units.
The sensing circuit is operative in an environment wherein at least one
control signal is generated by turning on and off an open-drain driver
connected through the first and second contacts to one end of a pull-up
resistor connected at its other end to a logic level voltage on the second
electronic module unit. Thus, when the first and second contacts are
connected together and the open-drain driver is turned off, the control
signal is pulled up to the logic level voltage by the pull-up resistor,
and when the open-drain driver is turned on, the control signal is driven
to ground through the open-drain driver.
The sensing circuit comprises a pull-down resistor connected between the
first contact and a ground reference voltage, and means for detecting a
voltage at the connection between the pull-down resistor and the first
contact. Thus, when the first and second contacts are connected together,
the pull-up resistor on the second electronic module unit and the
pull-down resistor on the first electronic module unit form a voltage
divider circuit between the logic level voltage on the second electronic
module unit and the ground reference on the first electronic module unit.
The resistance of the pull-down resistor is much larger than the
resistance of the pull-up resistor so that when the first and second
contacts are electrically connected together, a voltage at the connection
between the pull-up and pull-down resistors is substantially equal to the
logic level voltage. Hence, the pull-down resistor is termed "weak",
because when the first and second contacts are electrically connected
together, the pull-up resistor overpowers it and the voltage at the
connection between the pull-up and pull-down resistors is pulled up
towards the logic level voltage.
The weak pull-down resistor provides a means for sensing whether or not the
first and second contacts are electrically connected together. Assuming
that the open-drain driver is turned off at system start up or reset, the
voltage at the connection between the weak pull-down resistor and the
first contact is indicative of whether or not the first and second
contacts are electrically connected together. If the first and second
contacts are electrically connected together, then the pull-up resistor on
the second electronic module unit overpowers the weak pull-down resistor
on the first electronic module unit, and pulls the voltage up towards the
logic level voltage. If the first and second contacts are not electrically
connected together, then the weak pull-down resistor pulls the voltage
down towards the ground reference.
The addition of the weak pull-down resistor does not disturb the normal
function of the control signal when the first and second contacts are
electrically connected together. When the open-drain driver is turned off,
the pull-up resistor on the second electronic module unit overpowers the
weak pull-down resistor on the first electronic module unit, pulling the
control signal up to the logic level voltage. When the open-drain driver
is turned on, the control signal is pulled down to the ground reference
voltage through the open-drain driver.
Additional objects, features and advantages of the various aspects of the
present invention will become apparent from the following description of
its preferred embodiment, which description should be taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates, as an example, a top plan view of a mother board
including two slots adapted for receiving add-in boards;
FIG. 2 illustrates, as an example, an elevational view of a first type of
add-in board having a dual connector structure adapted for insertion into
a slot of a mother board;
FIG. 3 illustrates, as an example, an elevational view of a second type of
add-in board having a single connector structure adapted for insertion
into a slot of a mother board;
FIG. 4 illustrates, as an example, a system block diagram including the
mother board, and the first and second types of add-in boards;
FIG. 5 illustrates, as an example, a typical bus structure;
FIG. 6 illustrates a prior art circuit for sensing whether or not an add-in
board has been inserted into a bus connector of a mother board;
FIG. 7 illustrates, as an example, a first embodiment of a sensing circuit
utilizing aspects of the present invention; and
FIG. 8 illustrates, as an example, a second embodiment of a sensing circuit
utilizing aspects of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following description, the mother board and the add-in boards are
also generically referred herein as electronic module units, because each
of these boards is integrated into a unit carrying electronic modules
(i.e., integrated circuits). When an add-in board is referred to herein as
being inserted into a bus connector of a mother board, it is to be
understood that this means that one or more connectors of the add-in board
are mated to one or more corresponding connectors of the mother board so
as to permit the transmission of electrical signals through corresponding
contacts of the mated connectors. Where identical reference numbers are
used for blocks or components in the figures, similarly constructed and
functioning blocks or components are understood to be incorporated
therein.
FIG. 7 illustrates, as an example, a first embodiment of the present
invention. An add-in board 200, similarly constructed to the add-in board
20 of FIG. 2 (except for certain sensing circuitry utilizing aspects of
the present invention), is inserted into either slot 11 or 12 of the
mother board 10. If the add-in board 200 is inserted in slot 11, then
connectors 22 and 24 of the add-in board 200 are respectively inserted
into (i.e., mated with) connectors 15 and 16 of slot 11, and if the add-in
board 200 is inserted into slot 12, then connector 22 of the add-in board
200 is inserted into connector 17 of slot 12 and connector 24 of the
add-in board 200 is left "hanging in air".
A control line 50-y is connected between the add-in board 200 and the
mother board 10 when a contact 78 of the connector 24 of the add-in board
200 and a contact 80 of the connector 16 of the mother board 10 are
connected together (i.e., when connector 24 is inserted into connector
16). The control line 50-y is driven by an open-drain driver 68 (also
referred to herein as an open-collector driver) on the add-in board 200 in
conjunction with a strong pull-up resistor 62 on the mother board 10. The
pull-up resistor 62 is connected between a logic level voltage of 5 volts,
for example, and the contact 80 of the mother board 10, and the open-drain
driver 68 is connected to the contact 78 of the add-in board 200, such
that when the open-drain driver 68 is turned off, a voltage at a node 501
is pulled up close to the logic level voltage by the pull-up resistor 62,
and when the open-drain driver 68 is turned on, the voltage at node 501 is
driven towards ground (or a low voltage level) through the open-drain
driver 68.
In the present invention, a weak pull-down resistor 500 is connected
between node 501 and a ground reference (or low voltage reference), which
forms a voltage divider circuit with the strong pull-up resistor 62 of the
mother board 10, when the contact 78 of the add-in board 200 makes
electrical connection with the contact 80 of the mother board 10. The
pull-down resistor 500 is termed weak, because its value is substantially
larger than that of the pull-up resistor 62. In the preferred embodiment,
for example, the pull-up resistor 62 has a resistance value of 300
.OMEGA., and the pull-down resistor 500 has a resistance value of 22
K.OMEGA. (i.e., 22,000 .OMEGA.). Accordingly, when the contact 78 of the
add-in board 200 is making electrical connection with the contact 80 of
the mother board 10, the strong pull-up resistor 62 "overpowers" the weak
pull-down resistor 500 and pulls the voltage on the node 501 up close to
the logic level voltage, and when the contact 78 of the add-in board 200
is not making electrical connection with the contact 80 of the mother
board 10, the weak pull-down resistor 500 pulls the voltage on node 501
down close to the ground reference (or low voltage reference).
The voltage on node 501 can then be provided to other circuitry (not shown)
on the add-in board 200 via line 503, and/or driven back to a host
computer 40 of the mother board 10 via a data line 52-x, so that such
other circuitry (not shown) and/or the host computer 40 would know whether
or not the add-in board 200 has been inserted into slot 11 or 12, for
example, of the mother board 10. Preferably, data line 52-x is on the
8-bit data bus connected through connector 22 of the add-in board 200,
because the 8-bit data bus is available regardless of whether the add-in
board 200 is inserted into slot 11 or 12 of the mother board 10, whereas
the higher order data lines of the 16-bit data bus connected through
connector 24 of the add-in board 200 are only available if the add-in
board 200 is inserted into slot 11 of the mother board 10.
The voltage at node 501 is driven by tri-state driver 504 back to the host
computer 40 of the mother board 10 via data line 52-x, when the host
computer 40 causes the tri-state driver 504 to be enabled by addressing
the tri-state driver 504 via address lines 54. In particular, when
contacts 78 and 80 are electrically connected together and the voltage on
node 501 is pulled up towards the logic level voltage by the strong pull
up resistor 62, the tri-state driver 504 is driven to a logic level HIGH
when the host computer 40 enables the tri-state driver 504, and when the
contacts 78 and 80 are not electrically connected together and the voltage
on node 501 is pulled down towards ground by the weak pull-down resistor
500, the tri-state driver 504 is driven to a logic level LOW when the host
computer 40 enables the tri-state driver 504.
During normal mode operation, a signal generated by other circuitry (not
shown) on the add-in board 200 may be driven by tri-state driver 70 back
to the host computer 40 of the mother board 10 via data line 52-x, when
the host computer 40 causes the tri-state driver 70 to be enabled by
addressing the tri-state driver 70 via address lines 54.
Both tri-state drivers 70 and 504 are connected to data line 52-x of the
8-bit or 16-bit data bus through connector 22 of the add-in board 200. If
the function of control line 50-y ensures that it will not be driven LOW
by any add-in board (e.g., by open-drain driver 68 of add-in board 200)
when the host computer 40 of the mother board 10 reads the output of the
tri-state driver 504, then the output of the tri-state driver 504 is
indicative of whether the add-in board 200 has been inserted into either
slot 11 or slot 12 of the mother board 10. If the add-in board 200 has
been inserted into slot 11, then the output of the tri-state driver 504
will be a logic level HIGH. If the add-in board 200 has been inserted into
slot 12, then the output of the tri-state driver 504 will be a logic level
LOW. On the other hand, when the host computer 40 of the mother board 10
addresses the tri-state driver 70, it can read information provided by
other circuitry (not shown) on the add-in board 200 while operating in
normal mode operation.
FIG. 8 illustrates, as an example, a second embodiment of the present
invention. An add-in board 300, similarly constructed to the add-in board
30 of FIG. 3 (except for certain sensing circuitry utilizing aspects of
the present invention), is either inserted into slot 12 of the mother
board 10, or not inserted into any slot of the mother board 10 and
connected to a power supply in a stand-alone manner instead. Accordingly,
connector 32 of the add-in board 300 is either inserted into (i.e., mated
with) connector 17 of slot 12, or is left "hanging in air," if it is not
inserted into slot 12.
A control line 50-z known to be inactive/high at some time during normal
mode operation, is connected between the add-in board 300 and the mother
board 10 when a contact 92 of the connector 32 of the add-in board 300 and
a contact 94 of either the connector 15 or 17 of the mother board 10 are
connected together (i.e., when connector 24 is inserted into either slot
11 or 12 of the mother board 10). The control line 50-z is driven by an
open-drain driver 88 on the add-in board 300 in conjunction with a strong
pull-up resistor 96 on the mother board 10. The pull-up resistor 96 is
connected between a logic level voltage of 5 volts, for example, and the
contact 94 of the mother board 10, and the open-drain driver 88 is
connected to the contact 92 of the add-in board 300, such that when the
open-drain driver 88 is turned off, a voltage at node 506 is pulled up to
the logic level voltage by the pull-up resistor 96, and when the
open-drain driver 88 is turned on, the voltage at node 506 is driven to
ground (or a low voltage level) through the open-drain driver 88.
In the present invention, a weak pull-down resistor 505 is connected
between node 506 and a ground reference (or low voltage reference), which
forms a voltage divider circuit with the strong pull-up resistor 96 of the
mother board 10, when the contact 92 of the add-in board 300 makes
electrical connection with the contact 94 of the mother board 10. The
pull-down resistor 505 is termed weak, because its value is substantially
larger than that of the pull-up resistor 96. In the preferred embodiment,
for example, the pull-up resistor 96 has a resistance value of 300
.OMEGA., and the pull-down resistor 505 has a resistance value of 22
K.OMEGA. (i.e., 22,000 .OMEGA.). Accordingly, when the contact 92 of the
add-in board 300 is making electrical connection with the contact 94 of
the mother board 10, the strong pull-up resistor 96 "overpowers" the weak
pull-down resistor 505 and pulls a voltage on node 506 up close to the
logic level voltage, and when the contact 92 of the add-in board 300 is
not making electrical connection with the contact 94 of the mother board
10, the weak pull-down resistor 505 pulls the voltage on node 506 down
close to the ground reference (or low voltage reference).
The voltage on node 506 can then be preserved by providing it to the
high-active set input of an R-S latch 507, whose reset input is activated
by a system reset so that the RS-latch 507 is reset at power up, for
example. If the add-in board 300 is not inserted into slot 12 and the
open-drain driver 88 is turned off, then the pull-down resistor 505 pulls
node 506 down close to ground and the R-S latch 507 remains reset. If,
however, the add-in board 300 is inserted into slot 12 of the mother board
10 and the open-drain driver 88 (and any other drivers on any other add-in
boards connected to control line 50-z) is turned off, then the pull-up
resistor 96 will pull node 506 up close to the logic HIGH level voltage
and as a result, the R-S latch 507 is set. Thereafter, whenever a circuit
(not shown) on the add-in board 300 needs to know whether or not the
add-in board 300 has been inserted into slot 12 of the mother board 10,
the circuit (not shown) can check the contents of the R-S latch 507 by
enabling its output 508.
Although the various aspects of the present invention have been described
with respect to a preferred embodiment, it will be understood that the
invention is entitled to full protection within the full scope of the
appended claims.
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